There are various kinds of self-propelled machines for milling which are known. These machines include, in particular, road-milling machines, which can be used to remove existing layers of the surfacing structure of roads, and machines for working deposits by surface mining.
Road-milling machines have a milling arrangement which has a mechanically or hydraulically driven milling drum. The machines for working deposits by surface mining, which are referred to as surface miners, also have a milling arrangement having a milling drum, which is also referred to as a cutting drum in the case of a surface miner. The milling drums of road-milling machines and surface miners are fitted with tool holders to receive the milling tools.
The milling drums of road-milling machines or machines for working deposits are of a preset working width which corresponds to the width of the milling drum. When the machine is being used, the problem arises that the surfaces on which work is to be done are of a size whose width is a multiple of the working width of the machine. For work to be done on the entire surface, it is then necessary for a plurality of strips situated alongside one another to be milled. This calls for the machine to be accurately guided, because the individual milled strips have to be exactly aligned relative to one another. On the one hand the milled strips should extend alongside one another so close together that an unworked area is not left between the strips, but on the other hand the milled strips should not overlap, which would mean that productivity would be reduced. What is aimed for in practice is only a very small overlap, in order to ensure that, while productivity is high, work is done on the whole of the surface.
In the case of the known road-milling machines and machines for working deposits, what is done to work on a surface in a plurality of successive milled strips is that the driver of the machine strikes a visual balance between the dimensions of the machine and the surface to be worked on. Making this balance is a great strain on the driver and is tiring because, as well as the other jobs he has to do such as coordinating the unloading of the material picked up and doing the levelling, he also has to steer the machine exactly over the entire distance over which milling takes place. The driver is always faced with a conflict in this case, in that drifting into the strip milled previously will reduce efficiency but drifting in the opposite direction will result in work not being done on the whole of the surface. In the case of a milling machine which has an edge guard, there is also a risk of the machine being seriously damaged if the edge guard is pulled off.
U.S. Pat. No. 4,041,623 describes a track-laying road-milling machine which has provision for automatic presetting of the direction of travel. However, this presupposes that there is a tensioned cord which is fastened to posts which are arranged at a distance from one another along the desired strip to be milled. The arrangement for controlling the steering arrangement has a sensing arrangement having a sensing member which slides along the tensioned cord. The steering arrangement for steering the track-laying units is controlled in such a way that the machine follows the path of the cord exactly. It is true that the known road-milling machine allows the direction of travel to be controlled exactly but the need to have to tension a cord along the desired strip to be milled is found to be a disadvantage.
The object underlying the invention is to provide a road-milling machine or machine for working deposits which enables work to be done efficiently even on wide surfaces.
This object is achieved in accordance with the invention by virtue of the features of claim 1 the independent claims. Advantageous embodiments form the subject matter of the dependent claims.